Suppose the area of a circle is ${4\pi}$. What is its diameter?
Answer: ${K = 4\pi}$ ${d = 4}$ First, find the radius: $K = \pi r^2$ , so $r = \sqrt{K / \pi} = \sqrt{{4\pi} / \pi} = {2}$ Now find the diameter: $d = 2r = 2\cdot {2} = {4}$.